The present invention relates to an improvement in a dropout compensation circuit provided in a reproduction circuit to reproduce a signal recorded on a video tape or video disc, which compensates for a reproduction signal when dropout occurs during the reproduction of the same, that is, when the level of the reproduced signal is locally abruptly lowered.
The term "dropout" as used herein refers to a faulty portion of a detected signal, which is temporarily produced due to a defect in the information recording surface, and which lasts a brief period of time. Dropout is caused by scratches or dust on the recording medium, or the uneven quality of the information recording surface as occurs in the course of formation of the recording medium. The occurrence of dropout cannot be avoided. Dropout exerts a bad influence upon the reproduced signal and introduces unwanted noise thereinto. With respect to a video signal, the influence exerted thereupon can be reduced by using a luminance and/or chrominance signal delayed by a time corresponding to one horizontal line in place of the detected video signal where dropout occurs. However, in an audio signal which does not have a line-frequency structure, dropout compensation as is effected with the video signal is not available.
A dropout compensation technique for use with audio signals is disclosed in U.S. Pat. No. 3,989,893, according to which dropout of an audio signal being subjected to frequency modulation is compensated for prior to detection.
Another prior art technique exists which compensates for dropout of the information signal after the same is detected. FIG. 1 is a block diagram of a video disc player incorporating an information reproducing device of this kind. In the video disc player, video and audio information signals which are frequency-multiplexed are read out from a rotating video disc as optical variations or electrostatic capacitance variations by means of a pick-up 1.
The video and audio information signals read out therefrom are separated by means of band-pass filters 3 and 4 and supplied to a video demodulation circuit 5 and an audio demodulation circuit 6, respectively. The video information signal is demodulated in the demodulation circuit 5 and a composite video signal is provided therefrom, which is supplied to a video output terminal 7. On the other hand, the audio information signal is demodulated in a demodulation circuit 6. An audio signal is provided from circuit 6 which is then supplied to an audio output terminal 9 via a dropout compensation circuit 8. The dropout compensation circuit 8 receives a control signal fed from a dropout sensor 10, and thereby carries out dropout compensation when dropout occurs. Various types of dropout sensors are usable. For example, the dropout sensor may be constructed so as to sense dropout in the production of the high frequency noise component (ranging from 100 KHz to 200 KHz) derived from the audio demodulation circuit 6 or from the noise component contained in the output of the pick-up 1.
Such a dropout compensation circuit 8 is shown in FIG. 2, which circuit includes a low-pass filter 11 (having, for example, a cut-off frequency of 20 KHz) for removing an unwanted frequency component, two buffer amplifiers 12 and 13 of the voltage follower configuration, the gain of each is 1 (one), a switch 14, and a holding capacitor 15. Normally, the switch 14 is kept closed, so that the reproduction signal inputted to the low-pass filter 11 is output from the second buffer amplifier 13. In this condition, the output voltage of the low-pass filter 11 is held in the capacitor 15. When dropout occurs, the switch 14 is opened in response to the control signal fed from the dropout sensor 10 and the voltage being held in the capacitor 15 is taken from the output of the second amplifier 13 and supplied to the audio output terminal 9 while compensating for dropout. Signal waveforms at the points A, B and C of the circuit in FIG. 2 are shown in FIGS. 3A through 3C, respectively.
In this circuit, however, turbulence in the output of the low-pass filter 11 is prolonged when dropout occurs. Therefore, the holding time (T.sub.1 in FIG. 3B) of the capacitor 15 must be set to be a longer time. This causes distortion in the reproduced signal because the input waveform (FIG. 3A) of the low-pass filter 11 and the output waveform (FIG. 3C) of the second buffer amplifier 13 are different from one another with a relatively large amount of deviation.